by Richard Wolf, USA TODAY

by Richard Wolf, USA TODAY

WASHINGTON -- The Supreme Court will decide today the fate of two historic cases involving the marriage rights of gays and lesbians.

The nine justices, fresh off two days of rulings on decades-old racial issues, will complete their term by addressing an issue that barely existed just a decade ago.

One case, out of California, puts squarely before the court the question of whether same-sex couples can marry. The justices could say yes -- for Californians, gays and lesbians in several states, or nationwide. Or they could say no.

The other case, out of New York, will determine whether gay couples already married in a dozen states and the District of Columbia can receive federal benefits, ranging from spousal tax privileges to Social Security.

Depending on the results, hundreds of thousands of same-sex couples could be celebrating by midday -- or gay rights groups could be planning new campaigns to win rights accorded heterosexual men and women for centuries.

The rulings will be rendered on the 10th anniversary of the court's landmark Lawrence v. Texas decision that struck down state sodomy laws. That sweeping opinion by Justice Anthony Kennedy has given gay rights activists hope that he will follow suit by writing the decision that could strike down the federal Defense of Marriage Act.

The more historic ruling would be a sweeping declaration by the court that gay men and lesbians have the constitutional right to marry nationwide. Given the justices' hesitation to step on the prerogatives of states -- 36 of which have banned gay marriage -- that is considered unlikely.

Even a decision that legalizes gay marriage in California -- whether on the merits or one of several technicalities -- would boost from 18% to 30% the percentage of Americans living in states where gays and lesbians can marry.

Groups opposed to same-sex marriage, on the other hand, are hoping the justices will remember what several of them remarked upon during oral arguments in late March: that gay men and lesbians tying the knot is a more recent phenomenon than cellphones and the Internet. They have urged the court to let the debate play out in the states.

Several factors all appear to be breaking in favor of gay marriage as the justices prepare to render their verdicts:

The public increasingly is on the side of same-sex marriage, which could ease some justices' hesitance to get out in front of so many states. In the latest Pew Research Center survey, 72% of Americans called it "inevitable."

A half dozen states have legalized the practice since last fall: Maine, Maryland, Washington, Delaware, Rhode Island and Minnesota. Waiting in the wings are several others, from New Jersey to Hawaii.

Plaintiffs in the two cases arrived at the Supreme Court this year on a roll, having prevailed at federal district and appeals courts on opposite sides of the country.

The plaintiffs in the California case -- a gay couple and lesbian couple -- are expected to be in court Wednesday to witness the decision. The plaintiff in the New York case -- 83-year-old widow Edie Windsor -- will go before the cameras in Manhattan.